This invention relates to clutch teeth particularly for use in positive drive clutches or index couplings requiring high accuracy and where the parts undergo frequent engagement and disengagement. For example, tooth index couplings are used to obtain angularly spaced positions on rotary tables and tool index turrets. Such a coupling type is shown in U.S. Patent to Muller U.S. Pat. No. 2,202,117.
In use, the coupling members are tightly held in mesh by heavy springs or by hydraulic or pneumatic cylinders. To accomplish indexing the coupling members are axially separated and moved in some manner to a new location where the two coupling members are again squeezed tightly together. In general, the mechanism used to obtain the relative rotation is only approximate and as the coupling is engaged, there must be a slight rotation caused by the coupling teeth to reach the exact final location.
Very high accuracy is often required of these devices with some grades offered with less than 3 seconds of arc error. To retain this high accuracy, the prior art couplings for accurate indexing are made of hardened steel. Two such coupling designs are widely used. One is the "Curvio" curved-tooth coupling, and the other is the straight sided Hirth design. Neither of these couplings make use of the helicoid tooth form to obtain the desired accuracy. Both designs require a grinding operation after the coupling members are hardened. The hard surfaces are necessary, because as the teeth engage, there is only a theoretically very small area of contact and therefore, high stresses especially if fast operation and many operations are required.
To obtain better contact conditions on engagement coupling devices have been made which have a helicoid tooth form. For instance, Wildhaber shows in U.S. Pat. No. 2,654,456 a helicoid form of tooth chamfering. The clutch tooth design disclosed by Zieher in U.S. Pat. No. 2,950,797 attempts to improve conditions by combining a narrow band, which has flat engaging areas like the Hirth coupling with curved portions blended thereto. However, even with this design, there is still only contact at the extreme outside radius until the final engagement is reached. Moosmann, U.S. Pat. No. 3,820,412, shows a tooth form where the shape in the fully seated area is of helicoid form.
The term helicoid surface means a surface generated by a radial line which rotates about its axis and advances along the axis with a fixed relation between the two rates of motion.
We find that the helicoid tooth forms of the prior art are not suitable for a high accuracy coupling. The machinery necessary to produce the Moosmann design makes it very difficult to achieve the high precision required.